Turloughs are shallow depressions in a Carboniferous karst landscape, subject to periodic flooding, mainly from groundwater. They are most prevalent in the west of Ireland, although similar sites are found in Wales, Spain, Slovenia and eastern Canada. Hydrological pathways can connect several turloughs over large areas, which can fill and empty over short periods. The hydrology of turloughs can act as either a flow-through model, where inflow and outflow occur simultaneously, or a surcharged tank model, where the turlough acts as overflow storage for the underlying karst flow network, leading to greater residence time than in the through-flow model.
During the inundation phase, turloughs resemble and function like shallow lakes, but typically have no or few fish, that promotes persistence of large-bodied plankton and rare invertebrates. Amphibians, however, are common. During the winter, turloughs provide important habitat and feeding areas for wildfowl. Overall, phytoplankton tend to be dominated by cryptophytes, together with small diatoms. Shallow depth and wind-induced mixing results in suspension of algae that are predominantly associated with the vegetation on the turlough floor. Many turloughs are phosphorus-limited, although impact from greater intensification of agriculture is leading to increased phytoplankton populations in many sites. Being part of a karstic limestone catchment results in generally high calcium and carbonate concentrations of between 100 and 250 mg l−1 CaCO3. Some turlough catchments arising on acidic, non-limestone rocks overlain by upland blanket peat can have high concentrations of dissolved organic matter, and colour reaching over 80 mg l−1 PtCo.
The duration of flooding results in a characteristic zonation of turlough plant communities, but it is difficult to develop generalised models of vegetation zonation. Areas with long-duration flooding or permanent pools show characteristics of wetland communities, while the less-frequently inundated upper parts of a turlough show a transitions to grassland, scrub and woodland. During the dry phase, turloughs are often used as pastures for low intensity grazing, and are renowned for their diverse semi-wetland plant communities and associated invertebrates. Some turloughs dry out completely during dry summer periods, while others can retain some surface water throughout the year. Oligotrophic turloughs tend to have vegetation dominated by sedges with associated herbs, while those that are subjected to higher nutrient concentrations, particularly phosphorus, tend to be dominated by grasses and herbs.
Both the terrestrial and flooded phase contain ecological communities of national and international importance. Seasonal transitions between surface water and terrestrial systems results in a variable and species-rich ecotone and wetland mosaic habitat. This contributes to high conservation value, and many are protected as Special Areas of Conservation under European legislation. Yet, the hydrology of approximately one third of turloughs over 10 hectares had been irreversibly altered by drainage by the mid-1980s, and continuing pressures include removal of scrub and woodland, and reseeding of grassland with Lolium perenne, and direct application of fertiliser, or bulldozing the turlough basin to remove stones, with obvious degradation of vegetation communities. In recent years, there has been greater attention to their conservation, although many remain vulnerable to both localised and wider catchment pressures.
KeywordsTurlough Ephemeral Biodiversity Ireland Karst Groundwater Grazing
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